Graph Dynamical Networks for Unsupervised Learning of Atomic Scale Dynamics in Materials

Understanding the dynamical processes that govern the performance of functional materials is essential for the design of next generation materials to tackle global energy and environmental challenges. Many of these processes involve the dynamics of individual atoms or small molecules in condensed phases, e.g. lithium ions in electrolytes, water molecules in membranes, molten atoms at interfaces, etc., which are difficult to understand due to the complexity of local environments. In this work, we develop graph dynamical networks, an unsupervised learning approach for understanding atomic scale dynamics in arbitrary phases and environments from molecular dynamics simulations. We show that important dynamical information can be learned for various multi-component amorphous material systems, which is difficult to obtain otherwise. With the large amounts of molecular dynamics data generated everyday in nearly every aspect of materials design, this approach provides a broadly useful, automated tool to understand atomic scale dynamics in material systems.Comment: 25 + 7 pages, 5 + 3 figure

Lower Bounds of Concurrence for Tripartite Quantum Systems

We derive an analytical lower bound for the concurrence of tripartite quantum mixed states. A functional relation is established relating concurrence and the generalized partial transpositions.Comment: 10 page

Rate-dependent morphology of Li2O2 growth in Li-O2 batteries

Compact solid discharge products enable energy storage devices with high gravimetric and volumetric energy densities, but solid deposits on active surfaces can disturb charge transport and induce mechanical stress. In this Letter we develop a nanoscale continuum model for the growth of Li2O2 crystals in lithium-oxygen batteries with organic electrolytes, based on a theory of electrochemical non-equilibrium thermodynamics originally applied to Li-ion batteries. As in the case of lithium insertion in phase-separating LiFePO4 nanoparticles, the theory predicts a transition from complex to uniform morphologies of Li2O2 with increasing current. Discrete particle growth at low discharge rates becomes suppressed at high rates, resulting in a film of electronically insulating Li2O2 that limits cell performance. We predict that the transition between these surface growth modes occurs at current densities close to the exchange current density of the cathode reaction, consistent with experimental observations.Comment: 8 pages, 6 fig

An In Situ Surface-Enhanced Infrared Absorption Spectroscopy Study of Electrochemical CO2 Reduction: Selectivity Dependence on Surface C-Bound and O-Bound Reaction Intermediates

The CO_{2} electro-reduction reaction (CORR) is a promising avenue to convert greenhouse gases into high-value fuels and chemicals, in addition to being an attractive method for storing intermittent renewable energy. Although polycrystalline Cu surfaces have long known to be unique in their capabilities of catalyzing the conversion of CO_{2} to higher-order C1 and C2 fuels, such as hydrocarbons (CH_{4}, C_{2}H_{4} etc.) and alcohols (CH_{3}OH, C_{2}H_{5}OH), product selectivity remains a challenge. In this study, we select three metal catalysts (Pt, Au, Cu) and apply in situ surface enhanced infrared absorption spectroscopy (SEIRAS) and ambient-pressure X-ray photoelectron spectroscopy (APXPS), coupled to density-functional theory (DFT) calculations, to get insight into the reaction pathway for the CORR. We present a comprehensive reaction mechanism for the CORR, and show that the preferential reaction pathway can be rationalized in terms of metal-carbon (M-C) and metal-oxygen (M-O) affinity. We show that the final products are determined by the configuration of the initial intermediates, C-bound and O-bound, which can be obtained from CO_{2} and (H)CO_{3}, respectively. C1 hydrocarbons are produced via OCH_{3, ad} intermediates obtained from O-bound CO_{3, ad} and require a catalyst with relatively high affinity for O-bound intermediates. Additionally, C2 hydrocarbon formation is suggested to result from the C-C coupling between C-bound CO_{ad} and (H)CO_{ad}, which requires an optimal affinity for the C-bound species, so that (H)CO_{ad} can be further reduced without poisoning the catalyst surface. Our findings pave the way towards a design strategy for CORR catalysts with improved selectivity, based on this experimental/theoretical reaction mechanisms that have been identified

Assessing correlations of perovskite catalytic performance with electronic structure descriptors

Electronic structure descriptors are computationally efficient quantities used to construct qualitative correlations for a variety of properties. In particular, the oxygen p-band center has been used to guide material discovery and fundamental understanding of an array of perovskite compounds for use in catalyzing the oxygen reduction and evolution reactions. However, an assessment of the effectiveness of the oxygen p-band center at predicting key measures of perovskite catalytic activity has not been made, and would be highly beneficial to guide future predictions and codify best practices. Here, we have used Density Functional Theory at the PBE, PBEsol, PBE+U, SCAN and HSE06 levels to assess the correlations of numerous measures of catalytic performance for a series of technologically relevant perovskite oxides, using the bulk oxygen p-band center as an electronic structure descriptor. We have analyzed correlations of the calculated oxygen p-band center for all considered functionals with the experimentally measured X-ray emission spectroscopy oxygen p-band center and multiple measures of catalytic activity, including high temperature oxygen reduction surface exchange rates, aqueous oxygen evolution current densities, and binding energies of oxygen evolution intermediate species. Our results show that the best correlations for all measures of catalytic activity considered here are made with PBE-level calculations, with strong observed linear correlations with the bulk oxygen p-band center (R2 = 0.81-0.87). This study shows that strong linear correlations between numerous important measures of catalytic activity and the oxygen p-band bulk descriptor can be obtained under a consistent computational framework, and these correlations can serve as a guide for future experiments and simulations for development of perovskite and related oxide catalysts

Recent Insights into Manganese Oxides in Catalyzing Oxygen Reduction Kinetics

The sluggish kinetics of the oxygen reduction reaction (ORR) limit the efficiency of numerous oxygen-based energy conversion devices such as fuel cells and metal-air batteries. Among earth abundant catalysts, manganese-based oxides have the highest activities approaching that of precious metals. In this Review, we summarize and analyze literature findings to highlight key parameters that influence the catalysis of the ORR on manganese-based oxides, including the number of electrons transferred as well as specific and mass activities. These insights can help develop design guides for highly active ORR catalysts and shape future fundamental research to gain new knowledge regarding the molecular mechanism of ORR catalysis.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (award number DMR- 0819762)Skoltech-MIT CenterNational Science Foundation (U.S.). Graduate Research Fellowship (Grant no. DGE-1122374)United States. Department of Energy. Office of Basic Energy Sciences (contract no. DE-AC02- 98CH10886

Decarbonization of aviation via hydrogen propulsion: technology performance targets and energy system impacts

The aviation sector is challenging to decarbonize since aircraft require high power and energy per unit of weight. Liquid hydrogen is an interesting solution due to its high gravimetric energy density, minimal warming impact, and low-carbon production potential. We quantify the performance targets for fuel cell systems and on-board storage to enable hydrogen-powered regional aviation. We then explore the energy infrastructure impacts of meeting this additional H2 demand in the European context under deep decarbonization scenarios. We find that minimal payload reduction would be needed for powering regional aviation up to 1000 nmi if fuel cell system specific power of 2 kW/kg and tank gravimetric index of 50% can be achieved. The energy systems analysis highlights the importance of utilizing multiple technology options: such as nuclear expansion and natural gas reforming with CCS for hydrogen production. Levelized cost of liquid hydrogen as low as 3.5 Euros/kg demonstrates pathways for Europe to achieve cost-competitive production.Comment: 25 pages, 6 figures. (38 pages with SI, 7 SI figures

Salicylate method for ammonia quantification in nitrogen electroreduction experiments: The correction of iron III interference

[EN] The salicylate method is one of the ammonia quantification methods that has been extensively used in literature for quantifying ammonia in the emerging field of nitrogen (electro)fixation. The presence of iron in the sample causes a strong negative interference on the salicylate method. Today, the recommended method to deal with such interferences is the experimental correction method: the iron concentration in the sample is measured using an iron quantification method, and then the corresponding amount of iron is added to the calibration samples. The limitation of this method is that when a batch of samples presents a great iron concentration variability, a different calibration curve has to be obtained for each sample. In this work, the interference of iron III on the salicylate method was experimentally quantified, and a model was proposed to capture the effect of iron III interference on the ammonia quantification result. This model can be used to correct the iron III interferences on ammonia quantification. The great advantage of this correction method is that it only requires three experimental curves in order to correct the iron III interference in any sample provided the iron III concentration is below the total peak suppression concentration.This work was supported by the Toyota Research Institute through the Accelerated Materials Design and Discovery program. This work made use of the MRSEC Shared Experimental Facilities at MIT (SEM) supported by the National Science Foundation under award number DMR-1419807 as well as the HZDR Ion Beam Center TEM facilities. J.J.G.S. is very grateful to the Generalitat Valenciana and to the European Social Fund, for their economic support in the form of Vali+d postdoctoral grant (APOSTD-2018-001). G.M.L. was partially supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) PGS-D.Giner-Sanz, JJ.; Leverick, G.; Pérez-Herranz, V.; Shao-Horn, Y. (2020). Salicylate method for ammonia quantification in nitrogen electroreduction experiments: The correction of iron III interference. Journal of The Electrochemical Society. 167(13):1-10. https://doi.org/10.1149/1945-7111/abbdd6S11016713Kibsgaard, J., Nørskov, J. K., & Chorkendorff, I. (2019). The Difficulty of Proving Electrochemical Ammonia Synthesis. ACS Energy Letters, 4(12), 2986-2988. doi:10.1021/acsenergylett.9b02286Wang, Q., Guo, J., & Chen, P. (2020). The Power of Hydrides. Joule, 4(4), 705-709. doi:10.1016/j.joule.2020.02.008Wang, Y., Shi, M., Bao, D., Meng, F., Zhang, Q., Zhou, Y., … Jiang, Q. (2019). Generating Defect‐Rich Bismuth for Enhancing the Rate of Nitrogen Electroreduction to Ammonia. Angewandte Chemie International Edition, 58(28), 9464-9469. doi:10.1002/anie.201903969Andersen, S. 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Quantifying both ammonium and proline in wines and beer by using a PDMS composite for sensoring. Talanta, 198, 371-376. doi:10.1016/j.talanta.2019.02.001Prieto-Blanco, M. C., Jornet-Martínez, N., Moliner-Martínez, Y., Molins-Legua, C., Herráez-Hernández, R., Verdú Andrés, J., & Campins-Falcó, P. (2015). Development of a polydimethylsiloxane–thymol/nitroprusside composite based sensor involving thymol derivatization for ammonium monitoring in water samples. Science of The Total Environment, 503-504, 105-112. doi:10.1016/j.scitotenv.2014.07.077Prieto-Blanco, M. C., Ballester-Caudet, A., Souto-Varela, F. J., López-Mahía, P., & Campíns-Falcó, P. (2020). Rapid evaluation of ammonium in different rain events minimizing needed volume by a cost-effective and sustainable PDMS supported solid sensor. Environmental Pollution, 265, 114911. doi:10.1016/j.envpol.2020.114911McEnaney, J. M., Blair, S. J., Nielander, A. C., Schwalbe, J. A., Koshy, D. M., Cargnello, M., & Jaramillo, T. F. (2020). Electrolyte Engineering for Efficient Electrochemical Nitrate Reduction to Ammonia on a Titanium Electrode. ACS Sustainable Chemistry & Engineering, 8(7), 2672-2681. doi:10.1021/acssuschemeng.9b05983Schiffer, Z. J., Lazouski, N., Corbin, N., & Manthiram, K. (2019). Nature of the First Electron Transfer in Electrochemical Ammonia Activation in a Nonaqueous Medium. The Journal of Physical Chemistry C, 123(15), 9713-9720. doi:10.1021/acs.jpcc.9b00669Moliner-Martínez, Y., Herráez-Hernández, R., & Campíns-Falcó, P. (2005). Improved detection limit for ammonium/ammonia achieved by Berthelot’s reaction by use of solid-phase extraction coupled to diffuse reflectance spectroscopy. Analytica Chimica Acta, 534(2), 327-334. doi:10.1016/j.aca.2004.11.044López Pasquali, C. E., Fernández Hernando, P., & Durand Alegría, J. S. (2007). Spectrophotometric simultaneous determination of nitrite, nitrate and ammonium in soils by flow injection analysis. 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Coherent Pion Radiation From Nucleon Antinucleon Annihilation

A unified picture of nucleon antinucleon annihilation into pions emerges from a classical description of the pion wave produced in annihilation and the subsequent quantization of that wave as a coherent state. When the constraints of energy-momentum and iso-spin conservation are imposed on the coherent state, the pion number distribution and charge ratios are found to be in excellent agreement with experiment.Comment: LaTex, 8 text pages, 1 PostScript figure, PSI-PR-93-2